SubscriptionOnboardingStepPlugin

A single step in the native subscription onboarding flow, contributed from the feature module that owns the step (e.g. the VPN step lives in the VPN module, the Duck.ai step in the Duck.ai module). Each feature module contributes one implementation as a multibinding; the host in subscriptions-impl collects them, orders them (by @PriorityKey), and turns them into linear-onboarding steps.

Keeping this contract framework-agnostic (just a Fragment factory + an id) is what lets a step live in a different -impl module: the host never names the concrete Fragment class.